Update: Influenza Activity — United States and Worldwide, May 19–September 28, 2013

During May 19-September 28, 2013,* the United States experienced low levels of seasonal influenza activity overall. Influenza A (H1N1) pdm09 (pH1N1), influenza A (H3N2), and influenza B viruses were detected worldwide and were identified sporadically in the United States. In June, influenza A (H3N2) variant(†) viruses (H3N2)v were first detected in Indiana, and between June 18 and September 28, a total of 20 cases of influenza A variant viruses ([H3N2]v and influenza A (H1N1) variant [H1N1]v) were reported from five states. This report summarizes influenza activity in the United States and worldwide from May 19 through September 28, 2013.

* Data as of October 18, 2013. † Influenza viruses that circulate in swine are called swine influenza viruses when isolated from swine, but are called variant influenza viruses when isolated from humans. A variant virus (human isolate) might have the M gene alone or the M gene plus other genes from the influenza A (pH1N1) virus, along with other genetic changes. Seasonal influenza A (H3N2) viruses that circulate worldwide in the human population have significant antigenic and genetic differences from influenza A (H3N2) viruses circulating in swine. Additional information is available at http://www.who.int/ influenza/gisrs_laboratory/terminology_ah3n2v/en/index.html. § The CDC influenza surveillance system collects five categories of information from the eight data sources: 1) viral surveillance (World Health Organization collaborating laboratories, the National Respiratory and Enteric Virus Surveillance System, and human infections with novel influenza A viruses); 2) outpatient illness surveillance (U.S. Outpatient Influenza-like Illness Surveillance Network); 3) mortality (122 Cities Mortality Reporting System and influenza-associated pediatric mortality reports); 4) hospitalizations (FluSurv-NET, which includes the Emerging Infections Program and surveillance in five additional states); and 5) summary of the geographic spread of influenza (state and territorial epidemiologist reports). ¶ The 10 regions include the following jurisdictions: and/or sore throat, without a known cause other than influenza. † † The national and regional baselines are the mean percentage of visits for ILI during noninfluenza weeks for the previous three seasons plus two standard deviations. Noninfluenza weeks are defined as periods of 2 or more consecutive weeks in which each week accounted for <2% of the season's total number of specimens that tested positive for influenza. National and regional percentages of patient visits for ILI are weighted on the basis of state population. Use of the national baseline for regional data is not appropriate. § § The seasonal baseline proportion of P&I deaths is projected using a robust regression procedure in which a periodic regression model is applied to the observed percentage of deaths from P&I that were reported by the 122 Cities Mortality Reporting System during the preceding 5 years. The epidemic threshold is set at 1.645 standard deviations above the seasonal baseline. and one was associated with an influenza A virus for which subtyping was not performed. In all 20 cases, contact with swine in the week before illness onset was reported. No ongoing community transmission of these viruses has been detected. The median age of patients was 6.5 years (range: 2-69 years); 65% were female (Influenza Division, National Center for Immunization and Respiratory Diseases, CDC, unpublished data, 2013).

Worldwide
During May 19-September 28, typical seasonal patterns of influenza activity occurred in the temperate climate Southern Hemisphere countries. In Australia and New Zealand, influenza activity began in early August and decreased in mid-September. Influenza A viruses predominated in both countries with influenza A (H3N2) viruses identified more frequently than influenza A (pH1N1) viruses. Influenza B viruses were also identified in both countries. In South Africa, after a peak in influenza activity caused by influenza A (pH1N1) in June, a second, smaller peak was observed in early August because of increased influenza A (H3N2) and influenza B virus circulation. In temperate areas of South America, influenza activity peaked in June and declined through September. Influenza A viruses were reported more frequently than influenza B viruses, and influenza A (pH1N1) was the predominant virus reported by Argentina, Chile, and Uruguay. Influenza A (H3N2) viruses predominated in Paraguay (2).
Influenza activity was reported from countries with tropical influenza seasonality. The overall level of activity compared with previous seasons, and the predominant subtype varied by country. In the Caribbean and Central America, influenza activity peaked in early July and declined during August and September, with cocirculation of influenza A (pH1N1) and influenza A (H3N2) viruses. In tropical South America, influenza A (pH1N1) viruses predominated, with two peaks

Antigenic Characterization of Influenza Virus Isolates
The CDC antigenically characterized 342 influenza viruses collected during May 19-September 28 from laboratories worldwide, including 227 influenza A (pH1N1) viruses, 85 influenza A (H3N2) viruses, and 30 influenza B viruses. A subset of these viruses was genetically characterized. Most viruses tested were propagated in mammalian cell cultures; isolation rates of current human influenza viruses are higher in mammalian cell cultures than in eggs. However, egg-propagated (EP) vaccine viruses are used widely for production of influenza vaccines because most influenza virus vaccines are egg-based. Propagation of influenza viruses in eggs might lead to isolation of viruses that differ antigenically and genetically from viruses from corresponding clinical samples isolated in mammalian cell cultures. In addition, mammalian cell-propagated (CP) viruses are genetically more representative of viruses present in original clinical specimens (5,6). Therefore, it is important to select EP vaccine viruses that are antigenically and genetically most similar to their CP counterparts.
All but two (99%) influenza A (pH1N1) viruses analyzed (94 from North America, 131 from South America, one from Asia, and one from Oceania) were antigenically similar to

Editorial Note
During May 19-September 28, 2013, influenza A (pH1N1), influenza A (H3N2), and influenza B viruses cocirculated worldwide. In the United States, similar levels of seasonal influenza viruses were detected compared with summer months of previous years (excluding the 2009 pandemic), and influenza A viruses were predominant. Although neither the influenza viruses that will predominate nor the severity of influenzarelated disease during the 2013-14 season in the United States can be predicted, antigenic characterization of viral isolates submitted during the summer demonstrated that the majority of influenza viruses were antigenically similar to the influenza vaccine strains contained in the 2013-14 Northern Hemisphere vaccine.
Compared with the summer of 2012, fewer human infections with novel influenza A viruses were identified in the United States in the summer of 2013. Since the first identification of H3N2v viruses in humans, direct contact with swine has been documented in most cases, but limited person-to-person spread is suspected in a small number. Consistent with the age distribution of patients, serologic studies suggest there is little or no existing cross-reactive antibody to H3N2v in young children, but a substantial proportion of adolescents and younger adults have cross-reactive antibodies (8,9). Where community transmission of this virus has not been identified, the potential for this virus to develop the ability to transmit efficiently from person-to-person is of concern. Rapid and intensive investigation of each novel influenza A case remains necessary to evaluate the spread of disease and the possibility of personto-person transmission. While seasonal influenza viruses are circulating at low levels, state and local health departments should consider increased specimen collection among patients with ILI who 1) seek care at an ILINet provider; 2) are part of an ILI outbreak among children in child-care or school settings (because these settings were associated with person-to-person H3N2v, virus transmission in 2011); 3) have an unusual or severe presentation of ILI, including hospitalized persons; or 4) have medically attended ILI or acute respiratory infection, especially among children in jurisdictions where H3N2v cases have occurred (10).
Annual influenza vaccination remains the best method for preventing influenza and its associated complications (4). For optimal protection against seasonal influenza viruses, annual influenza vaccination is recommended for all persons aged ≥6 months each year, regardless of whether the vaccine virus strains have changed since the previous season. In addition to the types of influenza vaccines available during the last season, several new influenza vaccines have been approved for use and will be available for the 2013-14 season, including a quadrivalent live attenuated influenza vaccine (LAIV4), quadrivalent inactivated influenza vaccines (IIV4), a trivalent cell culture-based inactivated influenza vaccine (ccIIV3), and a new recombinant hemagglutinin vaccine (RIV3) (4). For many vaccine recipients, more than one type or brand of vaccine might be appropriate within indications and Advisory Committee on Immunization Practices (ACIP) recommendations. Where more than one type of vaccine is appropriate and available, ACIP has not recommended any one influenza vaccine product over another. Children aged 6 months-8 years who are being vaccinated for the first time should receive 2 doses of influenza vaccine. For children aged 6 months-8 years who have received influenza vaccination before, health-care providers should consult the ACIP guidelines to assess whether 1 or 2 doses are required (4).
What is already known on this topic? CDC collects, compiles, and analyzes data on influenza activity year-round in the United States. The influenza season generally begins in the fall and continues through the winter and spring months; however, the timing and severity of circulating influenza viruses can vary by geographic location and season.
What is added by this report?
The United States experienced low levels of influenza activity during May 19-September 28, 2013, and influenza A (H1N1) pdm09 (pH1N1), influenza A (H3N2), and influenza B viruses were identified sporadically. Twenty cases of influenza A variant viruses (influenza A [H3N2]v and influenza A [H1N1]v) were detected in five states, all of which were associated with swine contact. The majority of recent seasonal influenza viruses are antigenically similar to the influenza vaccine for the 2013-14 season.
What are the implications for public health practice?
To prevent influenza and its associated complications, influenza vaccination is recommended in all persons aged ≥6 months.
Year-round influenza surveillance provides critical information for planning interventions to prevent and control influenza, developing vaccine recommendations and antiviral treatment guidance, and presenting information to the media and public regarding the progress and severity of the influenza season.
Treatment with influenza antiviral medications is recommended as early as possible for patients with confirmed or suspected influenza (either seasonal influenza or variant influenza virus infection) who have severe, complicated, or progressive illness; who require hospitalization; or who are at higher risk for influenza-related complications (7). ¶ ¶ Influenza surveillance reports for the United States are normally posted online weekly and are available at http://www. cdc.gov/flu/weekly. Additional information regarding influenza viruses, influenza surveillance, influenza vaccines, influenza antiviral medications, and novel influenza A virus infections in humans is available at http://www.cdc.gov/flu.